1. Field of the Invention
The present invention relates to a grommet that reduces the insertion force to attach the grommet to a panel having an aperture, such as a vehicle body panel, while maintaining holding force after attachment to the panel.
2. Description of Related Art
A grommet made of an elastic material, including rubber and elastomer, is mounted on a wire harness routed from an engine compartment to a passenger cabin or a wire harness passed between a body panel and a door panel of a vehicle. The grommet is then attached to the panel at an aperture. The grommet is designed to protect the wire harness passed through the aperture, to shield the cabin from water, dust and noise from the engine compartment or to keep water and dust from entering the body panel or the door panel.
For a grommet of the above described type, it is required both to provide a low insertion force necessary to attach the grommet to the panel at the aperture and to provide a high holding or gripping force after the grommet has been attached. When attached to a wire harness routed between a body panel and a door panel in particular, the grommet is pushed into the aperture from the exterior, not pulled from inside the panel. Therefore, it would be an improvement to lower the insertion force in the pushing operation.
The applicant has presented in Japanese Patent Laid-open Publication No. 2001-132871 and others a grommet shown in FIGS. 7A–7C mounted on a wire harness routed between a body panel and a door panel and then pushed into an aperture in the body panel for attachment. Grommet 1 has elongated narrow tubular portion 2 and wide tubular portions 3 and 4 on ends thereof. One of the wide tubular portions fits into oval aperture 5a in body panel 5; the other wide tubular portion fits into an aperture in a door panel (not shown in the figure). Annular body latch groove 3a has a concave shape on an outer peripheral surface of wide tubular portion 3 (a body latch groove on wide tubular portion 4 is not shown in the figure). Groove bottom 3b of body latch groove 3a is sandwiched between two side walls. One of the side walls, side wall 3c on the insertion side of large tubular portion 3, has uniform height H from groove bottom 3b to upper end 3d along an entire periphery. When grommet 1 is pushed and latched into aperture 5a in body panel 5, upper end 3d of the side wall of wide tubular portion 3 passes through a periphery of aperture 5a and body latch groove 3a fits into the periphery of aperture 5a. 
When an aperture in a body panel is elongated, not round, wide tubular portion 3 of grommet 1 is also formed elongated as shown in a cross sectional view, similar to the shape of the aperture. Likewise, an annular body latch groove on wide tubular portion 3 is provided having an elongated shape. Therefore, side wall 3c of the body latch groove that catches the aperture includes a straight section and a circular arc section. When a pulling force is exerted on a wire harness having a grommet inserted therein and side wall 3c of the body latch groove latched into oval aperture 5a in body panel 5, the straight section of side wall 3c catching straight section 5a-2 disengages more easily from aperture 5a than the circular arc section of side wall 3c catching circular arc section 5a-1. A conventional grommet, therefore, has height H from groove bottom 3b to upper end 3d of side wall 3c catching the periphery of aperture 5a and predetermined so as not to allow side wall 3c to get loose in straight section 5a-2 and so as to be uniform along its entire periphery.
As described above, however, the section of the grommet catching circular arc section 5a-1 of the aperture is less likely to come loose than straight section 5a-2. Thus, the circular arc section of side wall 3c of the grommet has a latching and holding force that is greater than required. Furthermore, when inserting and latching wide tubular portion 3 of grommet 1 into oval aperture 5a, an excessive insertion force is required as the grommet has the side wall height in the circular arc section beyond that which is necessary. Normally, latching operations include one circular arc section of the body latch groove of the oval grommet is first latched into circular arc section 5a-1 of aperture 5a; then straight sections on both sides are latched to straight sections 5a-2 of the aperture; and finally the other circular arc section is inserted and latched into the circular arc section of the aperture. The final step that latches the circular arc section requires the maximum insertion force. Lowering the height of the side wall to reduce the grommet insertion force, however, decreases the holding force in the straight sections, leading to easy separation of the grommet from the panel having the aperture. Thus, conflicting issues arise herein: reducing the grommet insertion force weakens the holding force; maintaining the holding force does not reduce the insertion force. The issues apply not only to an oval aperture, but also to an aperture in a polygonal shape having a circular arc at each corner, to which the grommet is attached.